1. Field of the Invention
The present invention relates to methods of producing endoprosthetic devices, such as stents and stent-grafts, that are used to repair and/or treat diseased or damaged vessels and other structures within a body, and particularly to such devices that can be introduced at small delivery profiles and then enlarged in place.
2. Description of Related Art
Stents and stent-grafts are used in the treatment of vascular and other disease. They are particularly useful for treatment of vascular or arterial occlusion or stenosis typically associated with vessels narrowed by disease. Intraluminal stents and stent-grafts function to hold these vessels open mechanically. In some instances, they may be used subsequent to, or as an adjunct to, a balloon angioplasty procedure. Stent-grafts, which include a graft cover, are also particularly useful for the treatment of aneurysms. An aneurysm may be characterized as a sac formed by the dilatation of a wall or an artery, vein, or vessel. Typically the aneurysm is filled with fluid or clotted blood. The stent-graft provides a graft liner to reestablish a flow lumen through the aneurysm as well as a stent structure to support the graft and to resist occlusion or stenosis.
Treatment of a bifurcation site afflicted with such defects as an occlusion, stenosis, or aneurysm is a particularly demanding application for either stents or stent-grafts. A bifurcation site is generally where a single lumen or artery (often called the “trunk”) splits into two lumens or arteries (often called “branches”), such as in a “Y” configuration. For example, one such bifurcation site is found within the human body at the location where the abdominal aorta branches into the left and right (or ipsilateral and contralateral, respectively) iliac arteries.
When a defect, such as an aneurysm, is located very close to the bifurcation of a trunk into two branches, treatment becomes especially difficult. One reason for this difficulty is that neither the trunk nor either of the branches provides a sufficient portion of healthy vessel wall proximal and distal to the defect to which a straight section of single lumen stent or stent-graft can be secured. The stent or stent-graft must span the bifurcation site and yet allow relatively undisturbed flow through each of the branches and trunk.
Stents and stent-grafts offer considerable advantages over conventional surgery. Because they are comparatively less invasive, reduced mortality and morbidity, combined with shorter hospital stay, are the more significant advantages of stent and stent-graft therapies. Low profile endoprostheses (that is, endoprostheses that can be compacted into a small size for delivery) continue to be developed that enable the introduction of such devices through progressively smaller holes cut or punched through vessel walls. These low profile devices reduce blood loss and procedural morbidity compared to higher profile devices. Preferably, low profile devices should also be more flexible in the compacted delivery state. Devices that are more flexible during delivery better enable passage through tortuous vessels en route to the desired delivery site. Furthermore, thinner walled devices may cause less flow disturbance at the inlet and outlet to the graft.
The preferred device is one that can be introduced “percutaneously,” that is through a small transcutaneous incision or puncture 12 French (F) (4.0 mm) or less in diameter. Percutaneous delivery of a stent or stent-graft can often be done on an out-patient basis, and is typically associated with decreased patient morbidity, decreased time to patient ambulation, decreased procedural time, and potential reduction in health care delivery cost compared to surgical delivery of endoprostheses.
A “stent-graft” is formed by providing a covering on either the inside, outside, or both surfaces of the stent. These covered devices can provide a number of improvements over conventional uncovered stents. First, the cover may provide a fluid barrier (that is, either liquid or gas or both), prohibiting transmural fluid leakage from the inside to the outside of the device, or inhibiting transmural infiltration of fluids into the lumen of the device, or both. Second, covered devices can also limit tissue encroachment into the device over time. Third, it is believed that a covered device may provide an improved flow surface, which may aid in longer and more effective operating life for the device.
While covered stent devices have many benefits, unfortunately current covered-stents used for the treatment of disease of large vessels (e.g., thoracic or abdominal aortic vessels) generally require a surgical incision to provide a large enough access site to deliver such devices. Virtually all such devices currently are too large for less-invasive percutaneous delivery.
The current standard procedure for stent and stent-graft delivery is outlined below. The stent or stent-graft device is reduced in diameter (“compacted”) to enable it to be introduced through small incisions or punctures via a trans-catheter approach. “Self-expanding” devices inherently increase in diameter once a restraining mechanism is removed. “Restraining mechanisms” typically fit over part or all of the outer surface of compacted self-expanding devices to constrain them in a reduced diameter on the delivery catheter until deployment. “Deployment” is the term given to increasing the diameter of these intraluminal devices and subsequent detachment of the device from the delivery catheter. “Deployed inner diameter” as used herein is the device inner diameter measured immediately subsequent to releasing the device from its restraining mechanism in a 36–40° C. water bath and pressurizing the device to 1 Atm with an appropriately sized balloon dilatation catheter. An appropriately sized balloon will transmit the 1 Atm pressure to the device. For devices that cannot withstand a 1 Atm pressure, the deployed inner diameter corresponds to the size of the device immediately prior to rupture. For devices that require balloon expansion, the applied pressure is that pressure required to fully deploy the device to its intended dimensions.
Once self-expanding devices are properly positioned within the body, the restraining mechanism is removed, thereby deploying and anchoring the device. “Balloon-expandable” devices require the use of a balloon catheter or other means of dilatation within the recipient luminal structure for deployment and anchoring. Such devices are typically mounted and delivered on top of a balloon, which inherently increases their delivery profile.
As has been noted, percutaneous delivery is almost always preferred but is difficult or impossible to achieve for larger devices. A device (including restraining mechanism, if any) with a maximum outer dimension of no more than 10 French (F) (3.3 mm) can almost always be delivered percutaneously. More skilled physicians may opt to place devices percutaneously with dimensions of 12 F (4.0 mm), 13 F (4.3 mm), 14 F (4.7 mm), or more, although bleeding and other complications increase markedly with increasing access site size. Generally herein, a “percutaneous” device is considered to be a device that has an outer diameter in a delivered state of no more than 12 F.
Devices are generally placed into the body through percutaneously or surgically placed introducer sheaths that are sized according to their inner diameter. The wall thicknesses of these sheaths typically adds about 2 F to the size of the device. That is, a 12 F introducer sheath has about a 14 F outer diameter. “French” measurements used herein define the size of a hole through which a device will pass. For example, device with a measurement of “10 French” will pass through a 10 French hole (which has a diameter of 3.3 mm). A device need not have a circular cross-section in order to pass through a circular 10 French hole so long as the hole is large enough to accommodate the widest cross-section dimension of the device. The delivery size of an intraluminal stent-graft device is a function of stent geometry, stent-graft compacting efficiency, volume of the stent, volume of the stent cover, thickness of the restraining mechanism (if any), and the outer diameter of any guidewire or catheter within the lumen of the compacted device.
There are many problems encountered in attempting to compact a device into its smallest deliverable dimensions. First, the material of the stent element itself takes up a certain volume. If a graft component is added, this further increases the bulk of the device. Accordingly, there are absolute limits to compaction based strictly on the volume of the component parts.
Second, all known stent element designs provide the stent with crush-resistance (which is required if the stent is to have any structural value in holding open a vessel). This resistance to crushing further confounds attempts to tightly compact the device—with the risk that over-compacting the stent may damage its crush-resistance (and thus its structural value as a stent). On the other hand, less resilient stent devices might be more receptive to compaction, but are less effective in holding open the vessel once deployed.
Third, the graft material is also at risk of damage during compacting. Since the stent and the graft are compacted together, the stent element must be designed and compacted in such a way that it will not damage the graft when the two are compressed together.
Fourth, any compaction of a stent or stent-graft will likely tend to reduce the flexibility of the compacted device. Extreme compaction may produce a compacted device that is so inflexible that it will not negotiate tortuous paths in the body.
Fifth, as has been noted, currently available delivery devices and techniques (e.g., introducer sheaths, guidewires, delivery catheters, etc.) also add bulk to the device—generally adding about 2 to 3 F (0.67 to 1 mm) or more to the profile of the apparatus that must be delivered through the vascular access site.
Sixth, there are covered stents available today that can be compressed into small delivery profiles, but these devices undergo extreme elongation in their compressed state, with extreme foreshortening when transitioned to their deployed dimensions. These extremes in device length between compact and deployed dimensions make these devices difficult to properly position and deploy. Additionally, these devices tend to have less resilient stent structures. Finally, perhaps the greatest deficiency of these devices is that they must be covered with a material that can likewise undergo extreme elongation and contraction to match the longitudinal behavior of the stent element. As a result, preferred biocompatible graft materials such as polytetrafluoroethylene (PTFE) and woven DACRON® polyester are not readily used on these devices since neither is capable of extreme stretching and rebounding.
Results have been reported that a braided stent graft with a highly porous elastomeric covering allows the stent, when compacted for delivery, to be substantially elongated. Distributing the stent cross-sectional mass over a longer length (up to 40% length change) allows percutaneous delivery of a large device. Although these devices can fit through smaller delivery sites of 8 to 10 F, exact placement is often difficult because of the significant longitudinal retraction or recoil of the stent graft as it reaches its deployed size. The design of this stent relies on extreme elongation to achieve its compaction, hence undesirable foreshortening of the device naturally occurs during deployment. As a result, this design cannot accommodate a longitudinal strength member that would resist elongation during compaction of the device. In order to allow the stent frame to undergo extreme changes in length, elastomeric coverings are employed to permit the cover to expand and contract along with the stent frame. The coverings primarily serve as a barrier to the passage of blood and/or tissue or other elements in use, although their stretch and recovery requirements severely limit they types of materials that can be used in this device. Thickness and porosity is also a design limitation for these elastomeric stent coverings. To reduce porosity the coverings often must be thicker (often about 0.05 mm or greater), which can adversely effect delivery profile.
It should be evident from the above description that it is very desirable to provide an endoprosthetic device that can be delivered percutaneously. This is especially true for an endoprosthesis that combines the benefits of both a stent and a graft. However, there currently are a number of serious constraints limiting the ability to compact endoprosthetic devices, and particularly large vessel endoprosthetic devices (for instance, for treatment of aortic diseases and trauma), into their smallest possible delivery profiles.